The present invention relates to a method and apparatus to grow a single crystal of a dissociative compound semiconductor, controlling the pressure of a volatile component gas. Dissociative compound semiconductor includes, for example, III--V group compounds such as GaAs, InP, or GaP which are compounds of a volatile component (Vth group such as As or P) and a non-volatile second component (IIIrd group such as Ga or In). GaAs is taken hereinafter as an example, but the present invention is possible to apply for other dissociative compounds.
Japanese Patent Application Publication (kokai) No. 60-255692 discloses an example of an apparatus which is used in the Czochralski method for mono-crystalline growth of a dissociative compound semiconductor.
With reference to FIG. 1, the apparatus of the Japanese Application Publication for mono-crystalline growth has an inner air-tight vessel constituted by an upper vessel portion 1 with an observation scope 9 and a furnace 10 for controlling arsenic gas pressure, and a lower vessel portion 2. A circumferential joint portion 3, at which the upper vessel portion 1 and lower vessel portion 2 are joined, contains sealing material 7 such as B.sub.2 O.sub.3 which can be melted as a liquid seal at high temperature. Heaters 11 are installed so as to surround the air-tight vessel.
The inner vessel is supported on a pushing-up shaft 13 and a coil spring 8 which is inserted intermediately in the pushing-up shaft 13. A lower shaft 14 is installed through the pushing-up shaft 13, and has a crucible 4 which is laid on the end of the lower shaft 14. An upper shaft 5 penetrates an upper wall of the upper vessel portion 1.
Sealing portions 15 to keep the vessel air-tight are installed at a portion at which the upper shaft 5 slides through the upper vessel portion 1; and at a portion at which the lower shaft 14 slides through the lower vessel portion 2, so that the atmosphere within the inner vessel is isolated from the outer atmosphere. The sealing portions 15 consist of material such as B.sub.2 O.sub.3 which can be melted at high temperature.
With such construction, gallium is placed in the crucible 4. The whole apparatus is then evacuated and the inner vessel is sealed by pushing up the lower vessel portion 2. The heaters 11 are next turned on to vaporize the arsenic. Then, the temperature of the furnace 10, at which temperature is lowest on the inner vessel wall, is adjusted so that the vessel is filled with arsenic gas at a prescribed pressure. Arsenic gas reacts with gallium in the crucible 4, thereby GaAs melt is produced in the crucible 4.
A GaAs seed fixed at an end of the upper shaft 5 is dipped into the GaAs melt. The upper shaft 5 is pulled up while rotation of the shaft 14 about its axis. Consequently, a single crystal 6 of GaAs can be obtained.
During pulling up the upper shaft 5, a load cell (not shown) which is connected to the upper shaft 5 detects the change in the weight of the single crystal 6. The change of the weight is transmitted to a computer (not shown), which controls the power of the heaters 11 so as to control the diameter of the growing single crystal.
However, in this method, the upper shaft 5 receives an extra force from the difference between the inner pressure of the air-tight vessel and the pressure of the outer atmosphere. Such pressure difference arises when the two atmospheres partitioned by the inner vessel wall can not be balanced precisely and when a small periodical change of temperature exists on the inner vessel wall. And the output signal of the load cell corresponds to the summation of the true crystal weight and the extra force due to the pressure difference. If the pressure difference fluctuates, the output signal is spoiled by noise. It therefore becomes impossible to accurately control the crystal diameter.